Wireless communication systems enable users of User Equipment (UE) to communicate with other such users via one of a number of base stations and via one or a number of core networks. Typically, the UEs are mobile terminals, such as mobile (cellular) telephones and the like, although the term UE may also refer to generally stationary communication devices, such as laptop computers, web browsers, machine-type communication devices, and the like. In the following description the term user equipment is used, which is intended to cover any type of such user equipment (mobile and stationary).
In an active or connected state a UE is registered with the network and has a Radio Resource Control (RRC) connection with a base station so that the network knows to which base station (or cell thereof) the user communication device belongs and can transmit data to and receive data from the user communication device.
A Mobility Management Entity (MME) in the core network manages general mobility aspects of the UEs and ensures that connectivity is maintained with the UEs, for example as they are moving within the geographical area covered by the communication system (and/or as they are handed over between base stations of the communication system due to movement or changes in communication conditions). The MME also manages the various bearers associated with the UEs by controlling the other network nodes via which such bearers are provided. In order to do so, the MME exchanges Non-Access Stratum (NAS) signalling messages with the UEs (and/or the other network nodes) to manage the establishment of a communication session there between.